Cancer is still a huge social burden to society in terms of the hardship and suffering of patients and their loved ones, and also in terms of the high financial cost of treating, caring for and supporting patients. It is now thought that the immune system of healthy individuals clears cancerous cells routinely. However, in those patients with cancer one or more of the defense mechanisms involved in this clearance is/are down regulated or turned off completely.
It is now known that tumors change their microenvironment to make it more permissive to their growth. This occurs by the tumor releasing extracellular signals that, for example, promote tumor angiogenesis and/or induce local immune suppression or immune tolerance.
It is clear from many different preclinical and clinical studies that the microenvironment within tumours can suppress the development and activity of anti-tumour immune responses, with a wide variety of mechanisms being shown to potentially play a role. In particular immuno-suppressive mechanisms ultimately prevent T-cell responses from mediating the killing of tumour cells. Suppressive mechanisms may include the exclusion of T-cells from entering tumour tissues, inhibiting activation of T-cells that do enter the tumour and the modulation of tumour cell proteins which reduces the ability of T-cells to recognize or respond to them. The importance of such immunosuppressive pathways in supporting tumour progression has been particularly highlighted by the clinical efficacy shown by antibodies to receptors in two such suppressive pathways, CTLA4 and PD-1/PDL1, which has led to their marketing approval for the treatment of melanoma and other cancers.
B7 is a type of peripheral membrane protein found on activated antigen presenting cells (APC) that, when paired with either a CD28 or CD152 (CTLA-4) surface protein on a T cell, can produce a co-stimulatory signal or a co-inhibitory signal to enhance or decrease the activity of a MHC-TCR signal between the antigen presenting cell (APC) and the T cell, respectively. Besides being present on activated APCs, B7 can also be found on T-cells themselves.
There are several steps to activation of the immune system against an antigen. The T cell receptor must first interact with a complex of its specific peptide antigen (Ag) bound to a major histocompatibility complex (MHC) surface protein. The CD4 or CD8 proteins on the T-cell surface interact with the MHC to help stabilize the MHC/Ag interaction with the T-cell receptor complex, which comprises both the antigen-binding chain dimers (alpha/beta or gamma/delta) and the CD3 signaling complex (comprising gamma, delta, epsilon and zeta chains). This is also referred to as “Signal 1” and its main purpose is to provide the initial signaling and guarantee antigen specificity of the T cell activation.
However, MHC binding is insufficient by itself for stimulating full effector T cell differentiation and activation. In fact, lack of further stimulatory signals can render the T cell anergic. The co-stimulatory signals necessary to continue the immune response can come from B7-CD28 and CD40-CD40L interactions. There are other activation signals which play a role in immune responses. For example, in the TNF family of molecules, the protein 4-1BB (CD137) on the T cell may bind to 4-1BBL on the APC.
The B7 (CD80/B7-1 and/or CD86/B7-2) protein is present on the APC surface, and it interacts with the CD28 receptor on the T cell surface. This is one source of “Signal 2” (cytokines can also contribute to T-cell activation, which may be referred to as “Signal 3”). This interaction produces a series of downstream signals which promote the target T cell's survival, activation and differentiation into an effector cell that can mediate aspects of the immune response, such as killing of virus infected cells or tumour cells, and the recruitment of inflammatory cells.
Usually for initiating a T-cell response, the stimulatory signal and the co-stimulatory signal are provided by an antigen presenting cell in order to induce both CD4 and CD8 T-cell responses. But effector CD8 T-cells recognize their Ag associated with MHC class I molecules which are present on most nucleated cells, including tumour cells. However, the present inventors have reason to believe that the signals to activate T cells do not need to come from the same cell or cell type. Therefore it would be useful to provide one or more of these signals (i.e. the stimulatory signal and/or the co-stimulatory signal) to the immune system, for example on the surface of a cancer cell.
Currently there is much interest in inhibiting PD-1 (programmed cell death protein 1) and/or its ligand PDL1 (also known as B7-H1) activity because this pathway is thought to play an important role in down-regulating immune responses, for example in cancers.
However, some work done suggests that CD80 (B7-1) not only acts as a T-cell co-stimulator by binding to CD28 on the T-cell, it can also bind to PDL1, for example when expressed in the same cell membrane, and block PDL1-PD1 inhibitory signaling interactions. Thus, by acting in two different ways, CD80 may be a viable and potentially more useful molecule for restoring or enhancing the activation of human T cells. Soluble forms of CD80 also seem to be capable of counteracting PDL1-PD1 mediated T cell inhibition, see for example Haile et al Soluble CD80 Restores T Cell Activation and Overcomes Tumor Cell Programmed Death Ligand 1-Mediated Immune Suppression J Immunol 2013; 191:2829-2836. A CD80-Fc fusion protein has been generated and is being tested for safety and efficacy, see the Journal of Immunology, 2014, 193: 3835-3841.
The present inventors believe that the B7 proteins or an active fragment thereof delivered and expressed by an oncolytic virus, for example on the surface of a cancer cell, would be useful in activating the patient's own immune system to fight the cancer.
Furthermore, B7 proteins, such as CD80, if simply administered systemically have the potential to stimulate immune responses systemically in an undesirable way. The present inventors believe that a more sophisticated delivery of these proteins is required to create a suitable therapeutic window where beneficial therapeutic effects are realized and off target effects are minimized.